This invention relates to optical heating systems and more particularly to a method and apparatus for controlling the temperature of a region of material when heating with optical radiation.
Optical radiation is commonly used in many medical and industrial applications. One example of an optical radiation technique is using a laser welding procedure to seal wounds. In a laser welding procedure, edges of a wound are placed adjacent or in close approximity to each other and then heated by radiation from a laser at a wavelength of about 1.3 microns (near infrared region). The laser radiation is fed through an optical fiber and then directed at the wound. The tissue at the wound is then heated for a time duration sufficient to permit the tissue to denature and seal but not long enough to scorch. Examples of this procedure are described in more detail in U.S. Pat. Nos. 4,672,969, 4,854,320, 5,002,051, and 5,140,984.
When using the aforementioned procedures, a surgeon would direct laser energy from an emitter (optical fiber end) onto the area of the tissue to be heated. A spacing device is used to maintain a predetermined distance between the laser emitter and the material to be heated. The surgeon then activates the laser and heats the wound for a fixed time. The laser is then deactivated and the laser emitter is pointed to a new spot where the procedure is repeated. Critical to this procedure is maintaining the predetermined distance between the laser emitter and the tissue to be heated, as well as maintaining a constant power, intensity, and duration of the laser radiation. By doing so, the tissue being heated will reach the desired end temperature critical to the welding process.
A drawback to the aforementioned technique is that if the laser emitter is moved while the tissue is being heated such that the laser emitter becomes too close to the tissue, the tissue may overheat. This overheating may cause tissue scorching or shrinking and result in both scarring and an inadequate weld. If the laser emitter moves too great a distance from the tissue, the desired end temperature at which tissue denatures may not be reached and the wound may not close.
Another concern in tissue welding is that not all optical fibers through which the laser radiation is propagated to the wound are alike. Some fibers have greater attenuation than other fibers. Thus, certain fibers generate a small focal spot where another fiber may generate a large focal spot. These variations may cause the wound to heat faster or slower than desired.
One solution to overcome the problem of inconsistent heating is to use a thermocouple or a temperature probe. The thermocouple is placed on the area to be heated and measures the temperature rise of the area it contacts. A drawback of the thermocouple is that it may obscure at least part of the spot being heated. Consequently, the thermocouple can act as a heat sink and can interfere in the welding process. Further, the thermocouple may only measure temperature of a small portion of the spot being heated. Consequently, the thermocouple detects heat in one portion of the spot and not in other portions of the spot.
When organic tissue materials are heated, a gradient is typically formed with the hottest portion being in the center of the spot and coolest areas being around the spot's edges. Since the thermocouple is typically placed around the edges of the spot (to avoid obscuring the laser radiation), an accurate indication may not be provided of the temperature rise of the hottest portion of the tissue.
Another device procedure for heating with a laser is discussed in U.S. Pat. No. 4,121,087. This patent describes a technique that heats a metal reflective material with laser radiation and then senses the intensity of reflection of the laser radiation from the material. When heating tissue, the reflection characters may be modified with temperature. Therefore, when using this technique for laser welding, measuring the intensity of reflection from the tissue may result in inaccurate results.
Another device for controlling a laser is described in U.S. Pat. No. 4,316,467. This device optically senses hemangioma of skin and then regulates the laser depending on the color of the epidermis. Neither hemangioma nor color is related to tissue temperature and consequently would not be applicable when tissue welding.
A further device for laser surgery is described in U.S. Pat. No. 5,057,099. This device heats skin through an optical fiber and then monitors the heat at a radiation emitting end of the fiber. This heat on the end of the fiber may be unrelated to the final tissue temperature. As the fiber is moved, the tissue temperature changes but the temperature on the end of the fiber remains constant. Accordingly, this device would not be suited for tissue welding applications.